2 Answers
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PHP uses the external sendmail program to send E-mail messages (1, 2). To avoid installing it in its entirety in the chroot jail, lightweight programs exist that can provide the necessary functionality within the jail. How-tos for setting this up exist.

There is a small utility called "mail" or "email" or something like that, which can be installed as a simple replacement for sendmail, postfix, qmail, ... Just install to the chrooted environment and redirect the mails to the mailserver outside the jail.